Optical pickup

ABSTRACT

An optical pickup comprises a light source, a three-way split light diffracting section for dividing light emitted from a light source into one main beam and two subbeams, a hologram for dividing each of the main beam and the subbeams reflected from a recording medium into two predetermined directions, and a light receiving section for receiving the beams divided by the hologram. The hologram includes two regions having a small-pitch grating for diffracting each beam to a predetermined direction, the small-pitch gratings have substantially the same pitch and are symmetrical about a split line separating the two regions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical pickup used in anoptical disk system, such as a compact disk, a video disk, etc. Moreparticularly, the present invention relates to an optical pickupcomprising a hologram element-incorporating semiconductor laser device.

[0003] 2. Description of the Related Art

[0004] An optical pickup comprising a semiconductor laser device is usedto read out information stored in an optical disk, such as a compactdisk, etc. In the optical pickup, light emitted from the semiconductorlaser device is split by a diffraction grating of a hologram elementinto one main beam and two subbeams (tracking beams) which are broughtonto an optical disk. The main beam and the subbeams are reflected onthe optical disk, and each reflected beam is further split by a hologramof the hologram element into two beams, which are brought to a lightreceiving element or a signal processing integrated circuit with a lightreceiving element. Thereafter, based on an output signal from the signalprocessing integrated circuit, a tracking information signal, etc. usedfor accurately reading out signals recorded in the optical disk can beobtained.

[0005]FIG. 4 is a schematic diagram showing the optical system of aconventional three-beam hologram optical pickup.

[0006] This optical pickup has a semiconductor laser chip (LD) 6. Lightemitted from the semiconductor laser chip 6 is split by a tracking beamgenerating diffraction grating 5, provided on the rear side of ahologram element (not shown), into three beams, i.e., two subbeams fortracking and one main beam for reading information signals. This lightpasses through a hologram 4 provided on the hologram element aszero-order light, and is then converted by a collimator lens 3 toparallel light. The parallel light is condensed by an objective lens 2onto a disk 1. The light condensed onto the disk 1 is modulated by pitson the disk 1 and reflected from the disk 1. The reflected light fromthe disk 1 passes through the objective lens 2 and the collimator lens 3in this order, and is then diffracted by the hologram 4 and introducedinto a five-way split photodiode 7 as first-order diffracted light.

[0007] This five-way split photodiode 7 has five optical detectors D1 toD5. The five-way split photodiode 7 has a rectangular region which isilluminated by light. The region is divided into three equal parts whichare strip regions extending in a longitudinal direction. Two oppositeregions are first and fifth optical detectors D1 and D5. A middle stripregion is further divided into two equal parts in a transversedirection. One of the two regions is a fourth optical detector D4. Theother region is further divided into two parts in a longitudinaldirection, which are second and third optical detectors D2 and D3.

[0008] The hologram 4 has two regions 4 a and 4 b which have differentgrating pitches. The main beam of reflected light entering the region 4a is condensed onto the splitting line between the second opticaldetector D2 and the third optical detector D3 of the five-way splitphotodiode 7. The main beam of reflected light entering the region 4 bis condensed onto the fourth optical detector D4. Further, the twosubbeams of reflected light entering the region 4 a are condensed ontothe opposite first and fifth optical detectors D1 and D5, so that twobeam spots are formed on each of the optical detectors D1 and D5.

[0009] As described above, the beam spots of reflected light condensedon the optical detectors D1 to D5 of the five-way split photodiode 7vary depending on the focusing conditions of the light brought onto thedisk 1 as shown in FIGS. 5A to 5C. FIG. 5A shows spots when light isfocused beyond the optical disk 1. FIG. 5B shows spots when light isproperly focused on the optical disk 1. FIG. 5C shows spots when lightis focused before the disk 1.

[0010] The outputs of the optical detectors D1 to D5 of the five-waysplit photodiodes 7 are represented by S1, S2, S3, S4 and S5,respectively. A focus error signal FES is given by the differencebetween the outputs of the second optical detector D2 and the thirdoptical detector D3:

FES=S 2−S 3

[0011] A tracking error is detected by a so-called three-beam method.The tracking subbeams are condensed onto the optical detectors D1 andD5. A tracking error signal TES is given by the difference between theoutputs of the optical detectors D1 and D5:

TES=S 1−S 5

[0012] A reproduction signal RF is given by the sum of the outputs ofthe second, third and fourth optical detectors D2,D3 and D4:

RF=S 2+S 3+S 4

[0013] In ahologram optical pickup using the conventional three-beammethod, the hologram 4 includes two regions 4 a and 4 b having differentgrating pitches. Light beams which pass through the regions 4 a and 4 bof the hologram 4 after reflection on the optical disk 1 have differentdiffraction angles. Therefore, the light beams which have passed throughthe regions 4 a and 4 b are diffracted at a smaller angle and a largerangle in one direction with respect to the hologram 4.

[0014] The grating of the hologram 4 is typically formed of grooveswhich are formed by patterning using a photoetching technique. When thetwo regions 4 a and 4 b having different grating pitches are formed bypatterning, the depth of the grooves and the angle of grating vary ineach of the regions 4 a and 4 b, depending on an etching rate, etc.

[0015] If the groove depth and grating angle vary in each of the regions4 a and 4 b, the variations appear as the difference in the intensity ofdiffracted light between the main beam and the subbeams, i.e., thedifference in diffraction efficiency. As a result, the opticalintensities of reflected light beams entering the optical detectors D1to D5 are unbalanced, so that offset develops in the tracking errorsignal TES. In this case, characteristics of the hologram optical pickupare likely to be degraded.

SUMMARY OF THE INVENTION

[0016] According to one aspect of the present invention, an opticalpickup comprises a light source; a three-way split light diffractingsection for dividing light emitted from a light source into one mainbeam and two subbeams; a hologram for dividing each of the main beam andthe subbeams reflected from a recording medium into two predetermineddirections; and a light receiving section for receiving the beamsdivided by the hologram. The hologram includes two regions having asmall-pitch grating for diffracting each beam to a predetermineddirection, the small-pitch gratings have substantially the same pitchand are symmetrical about a split line separating the two regions.

[0017] In one embodiment of this invention, the light receiving sectionincludes a plurality of light receiving portions, the plurality of lightreceiving portions being symmetrical about a plane including a splitline separating the two regions and perpendicular to the hologram.

[0018] In one embodiment of this invention, the light receiving sectionhas a light receiving portion and the light receiving portion is notused to detect the beams.

[0019] In one embodiment of this invention, the plurality of lightreceiving portions of the light receiving section are positioned atsubstantially the same distance from the hologram.

[0020] In one embodiment of this invention, the light source, thethree-way split light diffracting section, the hologram and the lightreceiving section are integrated into the optical pickup.

[0021] Thus, the invention described herein makes possible theadvantages of providing an optical pickup in which the difference indiffraction efficiency between reflected light beams passing through tworegions provided in a hologram is reduced so as to reduce thedegradation of a characteristic of a tracking error signal TES.

[0022] These and other advantages of the present invention will becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic diagram showing the optical system of anoptical pickup according to the present invention.

[0024]FIG. 2 is a plan view showing an arrangement of optical detectorsof a five-way split photodiode according to the present invention.

[0025]FIG. 3 is a schematic diagram showing a specific configuration ofan optical pickup according to the present invention.

[0026]FIG. 4 is a schematic diagram showing the optical system of aconventional optical pickup.

[0027]FIG. 5A is a plan view of an arrangement of the optical detectorsin a conventional five-way split photodiode, showing spots when light isfocused beyond an optical disk.

[0028]FIG. 5B is a plan view of an arrangement of the optical detectorsin a conventional five-way split photodiode, showing spots when light isproperly focused on an optical disk.

[0029]FIG. 5C is a plan view of an arrangement of the optical detectorsin a conventional five-way split photodiode, showing spots when light isfocused before an optical disk.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

[0031]FIG. 1 is a schematic diagram showing the optical system of anoptical pickup according to the present invention.

[0032] This optical pickup comprises a semiconductor laser chip 16 whichemit predetermined light. Light emitted from the semiconductor laserchip 16 is split by a tracking beam generating diffraction grating 15provided on the rear side of a hologram element (not shown) to threebeams, i.e., two subbeams for tracking and one main beam for readinginformation signals. Those three light beams pass through a hologram 14provided on the hologram element as zero-order light, and are thenconverted by a collimator lens 13 to parallel light. The parallel lightis condensed by an objective lens 12 onto a disk 11. The light condensedonto the disk 11 is modulated by pits formed on the disk 11 andreflected from the disk 11. The reflected light from the disk 11 passesthrough the objective lens 12 and the collimator lens 13 in this order,and is then diffracted by the hologram 14 and introduced into a five-waysplit photodiode 17 as first-order diffracted light.

[0033] The hologram 14 is in the shape of a circle, and is divided alonga split line on a diameter of the circle into two equal semicircularregions 14 a and 14 b. The regions 14 a and 14 b include small-pitchgratings having substantially the same grating pitch. The small-pitchgratings provided in the regions 14 a and 14 b are arranged to besubstantially symmetrical about the split line separating the regions 14a and 14 b. Therefore, the small-pitch gratings of the regions 14 a and14 b have substantially the same pitch. Therefore, the reflected lightbeams passing through the regions 14 a and 14 b of the hologram 14 arediffracted at substantially the same angle toward substantially the samedirection, so that the light beams are substantially symmetrical about aplane which includes the split line separating the regions 14 a and 14 band is perpendicular to the hologram 14. Therefore, the light beamspassing through the regions 14 a and 14 b of the hologram 14 areseparated into two directions as first-order diffracted light beams.

[0034]FIG. 2 is a plan view showing an arrangement of optical detectorsD1 to D5 of the five-way split photodiode 17 which reflected light beamsenter.

[0035] The main beam and the two subbeams are each split intofirst-order diffracted light beams propagating in two directions by thetwo regions 14 a and 14 b of the hologram 14. The optical detectors D1to D5 is arranged by considering the incident positions of the firstdiffracted light beams. The first to fifth optical detectors D1 to D5are arranged on the same plane and along a direction orthogonal to thesplit line splitting the hologram 14 into the two regions 14 a and 14 b.The first and fifth optical detectors D1 and D5 onto which two subbeamspassing through the region 14 a are brought are positioned on theopposite sides of the photodiode 17. A pair of the second and thirdoptical detectors D2 and D3, onto which the main beam passing throughthe region 14 a of the hologram 14 is brought, are provided next to thefirst optical detector D1. The fourth optical detector D4, onto whichthe main beam passing through the region 14 b of the hologram 14 isbrought, is provided next to the fifth optical detector D5. The firstand fifth optical detectors D1 and D5 are in the shape of aparallelogram extending in a direction along which the subbeams arediffracted, and are substantially symmetrical about a plane whichincludes the split line of the hologram 14 and is perpendicular to thehologram 14. The distance between the first and fifth optical detectorsD1 and D5 is increased the further their positions from the hologram 14.The fourth optical detector D4 provided next to the fifth opticaldetector D5 is in the shape of a parallelogram similar to the fifthoptical detector D5. The second and third optical detectors D2 and D3are in the shape of parallelograms obtained by dividing a parallelogram,which is substantially symmetrical to the fourth optical detector D4about a plane which includes the split line of the hologram 14 and isperpendicular to the hologram 14, into substantially two equal parts ina longitudinal direction.

[0036] Further, an optical detector Dx is provided between the third andfourth optical detectors D3 and D4, which is used to prevent theoccurrence of stray light, which affects the outputs of the opticaldetectors D1 to D5. The optical detector Dx does not detect the mainbeam and subbeams. The optical detector Dx is in the shape of atrapezoid such that the width is broadened the further its position fromthe hologram 14. The position of the optical detector Dx is such thatthe optical detector Dx seemingly buries a region between the third andfourth optical detectors D3 and D4.

[0037] The longitudinal length of each optical detector D1 to D5 isdesigned to be greater than a range within which the incident positionof reflected light varies depending on the wavelength which varies asthe temperature of a light source is changed, so that the desiredoutputs of the optical detectors D1 to D5 can be obtained. It should benoted that if the lengths of the optical detectors D1 to D5 are longerthan that required, the capacitive components of the optical detectorsD1 to D5 are large, leading to a reduction in response speed. Thelengths of the optical detectors D1 to D5 are determined by consideringthe precision of positioning. Further, since each of the opticaldetectors D1 to D5 is in the shape of a parallelogram, a wastage ofspace is reduced when compared to the case where each detector is in theshape of a rectangle extending along the same direction as that of theparallelogram. Therefore, peripheral circuits, such as an amplifyingcircuit, etc., can be easily arranged.

[0038] The main beam of reflected light from the disk 11 is passedthrough the regions 14 a and 14 b of the hologram 14 so that the mainbeam is diffracted into directions substantially symmetrical about thesplit line separating the regions 14 a and 14 b and the split beams formrespective beam spots between the second and third optical detectors D2and D3 of the five-way split photodiode 17, and the fourth opticaldetector D4. The two subbeams are passed through the regions 14 a and 14b of the hologram 14 so that the subbeams are diffracted into directionssubstantially symmetrical about the split line separating the regions 14a and 14 b and the split subbeams form respective beam spots on thefirst and fifth optical detectors D1 and D5 on the opposite sides of thefive-way split photodiode 17.

[0039] As described above, in the optical pickup of the presentinvention, the hologram 14, which divides reflected light from theoptical disk 11 into two parts, has two grating regions 14 a and 14 bwhose grating pitches are substantially equal to each other. Therefore,the variations in the depth of the small-pitch grating and the angle ofthe grating between the regions 14 a and 14 b can be reduced, so thatthe differences in the intensity of the diffracted light (i.e.,diffraction efficiency) of the main beam and the subbeams can be reducedbetween the regions 14 a and 14 b. As a result, characteristics of theoptical pickup can be prevented from being degraded. Particularly, it ispossible to improve the balance characteristic of a tracking errorsignal TES.

[0040]FIG. 3 is a schematic diagram showing a specific configuration ofan optical pickup according to the present invention.

[0041] This optical pickup has a plate-like stem 20 which is supportedvia a plurality of lead pins 21 by an optical disk apparatus body. Asemiconductor laser chip 16 and a five-way split photodiode 17 aremounted on a surface of the stem 20. The semiconductor laser chip 16 andthe five-way split photodiode 17 are connected via wiring (not shown) tothe optical disk apparatus. A hollow cap 19 is provided on the top sideof the stem 20 f or covering the semiconductor laser chip 16 and thefive-way split photodiode 17 to block light. A hologram element 22 isprovided on the top wall of the cap 19. A tracking beam generatingdiffraction grating 15 is provided on the lower side of the hologramelement 22. Ahologram 14 is provided on the upper side of the hologramelement 22.

[0042] The thus-obtained optical pickup is a package into which thehologram element 22, the semiconductor laser 16, the photodiode 17, etc.are integrated, resulting in miniaturization of the optical pickup.Further, the production processes of such an optical pickup can besimplified.

[0043] In the optical pickup of the present invention, the hologram hastwo regions having a small-pitch grating which diffracts light into apredetermined direction. The small-pitch gratings have substantially thesame pitch and are substantially symmetrical about a split lineseparating the two regions. Therefore, variations in the depth and angleof the small-pitch gratings can be reduced between the two regions. Inthis case, the differences in the intensity of the diffracted light (i.e., diffraction efficiency) of a main beam and subbeams can be reducedbetween the two regions. Therefore, characteristics of the opticalpickup can be prevented from being degraded. Particularly, it ispossible to improve the balance characteristic of a tracking errorsignal TES.

[0044] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical pickup, comprising: a light source; athree-way split light diffracting section for dividing light emittedfrom a light source into one main beam and two subbeams; a hologram fordividing each of the main beam and the subbeams reflected from arecording medium into two predetermined directions; and a lightreceiving section for receiving the beams divided by the hologram,wherein the hologram includes two regions having a small-pitch gratingfor diffracting each beam to a predetermined direction, the small-pitchgratings have substantially the same pitch and are symmetrical about asplit line separating the two regions.
 2. An optical pickup according toclaim 1, wherein the light receiving section includes a plurality oflight receiving portions, the plurality of light receiving portionsbeing symmetrical about a plane including a split line separating thetwo regions and perpendicular to the hologram.
 3. An optical pickupaccording to claim 2, wherein the light receiving section has a lightreceiving portion and the light receiving portion is not used to detectthe beams.
 4. An optical pickup according to claim 2, wherein theplurality of light receiving portions of the light receiving section arepositioned at substantially the same distance from the hologram.
 5. Anoptical pickup according to claim 1, wherein the light source, thethree-way split light diffracting section, the hologram and the lightreceiving section are integrated into the optical pickup.